The focus of this project is the study of immune responses generated by vectors including live viral vectors and recombinant plasmid DNA. Particular subjects under study include defining the mechanism of immune responses generated by various vectors and their effects on subsequent reexposure to the same or different vectors; testing plasmid DNA vaccines for potency and efficacy, as well as mechanisms of action; and testing mice with targeted disruption of genes causing various partial immune deficiencies for their ability to respond to viral and plasmid vectors. Viral and plasmid vectors are currently being tested for two major uses in human health care: as vaccines and as gene therapy vectors. The induction of immune responses to gene products expressed by a vector determines the success of vaccination. Immune mechanisms responsible for protection by some vaccines are not well understood, and some immune responses can even be harmful, leading to more severe disease upon subsequent exposure. In the case of use of vectors for gene therapy, immune responses can be a limiting factor, especially in the context of readministration. Such immune responses can block efficacy, as seen in clinical trials already, or can cause immunopathology. Immunity to the transgene product of a gene therapy vector could also result in autoimmune-like pathology in some cases. Better understanding of vector-induced immune responses, as studied in this program, can help select predictors of clinical adverse events, and thus can contribute to improved regulatory decision-making. Better definition of the specificity and potency of responses to these products, and identification of the immune mediators that are protective or harmful in specific systems, will help in evaluation of new viral vaccines and gene therapy vectors.